The present invention relates to a rubber composition suitable as a material of an anisotropically electroconductive membrane or, more particularly, to a rubber composition suitable as a material of an anisotropically electroconductive membrane having excellent flexibility and high heat resistance. The invention also relates to an anisotropically electroconductive membrane shaped from the composition.
An anisotropically electroconductive membrane is a sheet consisting of a polymeric material as the matrix and electroconductive fine particles dispersed therein and serves by being sandwiched between two arrays of electrodes each on an electronic device such as LSIs, liquid-crystal display panels and the like for electrically connecting the oppositely facing electrodes on both sides keeping insulation between adjacent electrodes on the same side. By virtue of simplicity of the assembling works of an electronic instrument, more and more electronic instruments are being assembled by using anisotropically electroconductive membranes. It is also desirable that an anisotropically electroconductive membrane has good adhesiveness to serve as an adhesive layer for adhesively bonding two electronic elements on both sides.
Most conventionally, anisotropically electroconductive membranes are prepared from a composition comprising a thermoplastic resin and fine metal particles dispersed in the polymeric matrix. These thermoplastic resin-based anisotropically electroconductive membranes are not quite satisfactory in respect of the relatively low heat resistance and poor adhesiveness. Alternatively, thermosetting resin-based compositions are proposed as a material of anisotropically electroconductive membranes (see, for example, Japanese Patent Kokai No. 61-195569, No. 62-271273 and No. 63-316885). The thermosetting resin-based anisotropically electroconductive membranes, however, are disadvantageous because the heat resistance thereof is not high enough to durably withstand the temperature of molten solder alloy used in the assemblage work of electronic instruments and the storage stability of thermosetting resins is usually not high enough to cause a difficulty in ensuring a constant curing velocity at a specified curing temperature.
It would be a possible way to improve an anisotropically electroconductive membrane relative to the heat resistance against molten solder alloys and storage stability by using, as the matrix polymer, a rubbery fluorocarbon polymer well known to be heat-resistant and flexible at room temperature. Conventional rubbery compositions based on a fluorocarbon polymer, however, are not very satisfactory for the purpose because good storage stability and good curability are inconsistent with each other sot that a fluorocarbon polymer-based rubbery composition having good curability is usually not fully stable in storage and vice versa. A fluorocarbon polymer-based rubbery composition compounded with an organic peroxide as a curing agent in place of an amine compound or polyol compound may have good curability along with excellent storage stability but such a composition has no adhesiveness so that anisotropically electroconductive membranes as desired cannot be prepared from such a composition.